Recent experiments suggest that there might be an endogenous mammalian analogue to the digitalis glycosides which would regulate the mammalian Na+/K+-ATPase (Na+ pump) in a physiologic rather than pharmacologic way. Such a compound has been linked to control of renal Na+ excretion, positive inotropic effects in cardiac muscle, and may play a role in the pathogenesis of a prevalent human disease, hypertension. A high affinity, specific, reversible inhibitor of the mammalian Na+/K+-ATPase has been completely purified from bovine hypothalamus. This hypothalamic inhibitory factor, HIF, has biological properties similar to, but not identical to, those of the cardiac glycoside, ouabain, and consistent with physiologic regulation in vivo. Physiochemical characterization of HIF indicates that while both ouabain and HIF are found to be alpha-L rhamnosides, they are structurally different in their aglycone portions. This structural difference is presumed to account for the observed differences in the biological properties of HIF and ouabain. The aims of the current proposal are (1) to purify several micrograms of HIF, utilizing established procedures; (2) to use the pure HIF to determine by liquid chromatography/mass spectrometry and circular dichroism spectrometry whether the aglycones of ouabain and HIF are stereochemically identical; (3) if they are not identical, to use further spectroscopic studies to assign the sterochemistry of the HIF-aglycone; and (4) if they are identical, to synthesize position isomers testing the products for biological activity. Success in this work would define the specific structural difference between the pharmacologic inhibitor, ouabain, and the putative mammalian physiologic analogue, HIF. Knowing the exact structure of HIF is desirable since it would probably reveal a general strategy for improving the therapeutic index of cardiac glycosides, and provide a probe to study the proposed role of endogenous Na+/K+-ATPase inhibition in the pathogenesis of human hypertensive disease.